Overlay system with digital optical transmitter for digitized narrowcast signals

ABSTRACT

Methods and apparatuses are provided to modify existing overlay system architectures in a cost effective manner to meet the growing demand for narrowcast services and to position the existing overlay systems for additional future modifications. The implementations of the improved overlay system of this disclosure re-digitize narrowcast analog signals after they have been QAM modulated and upconverted to RF frequencies and replace the analog narrowcast transmitters with digital narrowcast transmitters. In the fiber nodes, the received narrowcast signals are converted back to analog signals and combined with analog broadcast signals for transmission to the service groups.

TECHNICAL FIELD

This disclosure relates to an overlay system.

BACKGROUND

A cable-based system can be used to deliver high-definition digitalentertainment and telecommunications such as video, voice, andhigh-speed Internet services from a headend to subscribers over anexisting cable television network. The cable television network can takethe form of an all-coax, all-fiber, or hybrid fiber/coax (HFC) network.

Generally, analog video signals and digital bit streams representingvarious services (e.g., video, voice, and Internet) from various digitalinformation sources are received at the headend and converted to radiofrequency (RF) optically modulated signals for transmission over thecable network. One or more services output from the headend can occupy aspecific 6 MHz-wide RF channel having a center frequency that typicallyfalls within a frequency range having a lower limit of 50 MHz and anupper limit of 1002 MHz.

For digital bit streams, in some implementations, each of the digitalbit streams is encoded to produce a corresponding digital QAM symbolstream. Each digital QAM symbol stream is root-nyquist filtered,converted to an analog QAM symbol stream, and QAM modulated onto an RFcarrier signal having a frequency that corresponds to a center frequencyof a 6 MHz-wide RF channel. For digital broadcast services (e.g.,service that are intended for all subscribers in a serving area) such asvideo, the RF carrier signal frequency typically falls within afrequency range having a lower limit of 550 MHz and an upper limit of750 MHz. For digital narrowcast services (e.g., services that areintended for a single customer in a serving area) such as video ondemand, internet data, and telephony, for example, the RF carrier signalfrequency can vary from system to system. Broadcast analog video signalsreceived at the headend are modulated onto an RF carrier signal having afrequency that typically falls within a frequency range having a lowerlimit of 50 MHz and an upper limit of 550 MHz.

Some or all of the analog single-channel modulated RF carrier signalscan be combined to produce an analog multi-channel RF signal. One ormore analog optical transmitters convert the analog single-channelmodulated RF carrier signals and/or multi-channel RF signals tooptically modulated signals.

Generally, the RF optically modulated signals are transmitted from theheadend via one or more fibers to one or more fiber nodes. Each of thefiber nodes includes an optical receiver that converts the receivedoptically modulated signals representing broadcast and narrowcastservices to electrical RF signals. The electrical RF signals then aretransmitted to receiving devices such as cable modems (CMs) and/orsettop boxes (STBs) that are served by the fiber node. All of thereceiving devices served by the fiber node can receive the electrical RFsignals. If the electrical RF signal represents a broadcast service,each receiving device served by the fiber node can process and deliverthe corresponding service to the subscriber. If the electrical signalrepresents a narrowcast service, the receiving device to which theelectrical signal is addressed can process and deliver the correspondingservice to the subscriber.

In a so-called overlay system, an analog broadcast optical transmittertransmits broadcast RF optically modulated signals on a first fiber. Fornarrowcast services, for each fiber node there can exist an analognarrowcast optical transmitter in the headend to output narrowcast RFoptically modulated signals at a particular wavelength designated forthe fiber node. The narrowcast RF optically modulated signals output byan analog narrowcast optical transmitter can comprise one or more RFchannels. A multiplexer combines the narrowcast RF optically modulatedsignals produced by the narrowcast optical transmitters to produce amulti-wavelength RF optically modulated signal on a second fiber. Thebroadcast RF optically modulated signal transmitted on the first fiberand the multi-wavelength RF optically modulated signal transmitted onthe second fiber can be received at an optical transition node (“OTN”).At the OTN, the narrowcast signals are demultiplexed by an opticaldemultiplexer. For each narrowcast signal output from the demultiplexer,an optical combiner combines the broadcast signal and the narrowcast,and the resulting signal is transmitted to the designated fiber node fordelivery to the receiving devices as discussed above.

There is a growing demand for narrowcast services; however, the existingoverlay system architecture is not adequate to meet the growing demandfor narrowcast services.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example cable-based system.

FIG. 2 is a block diagram illustrating an example processing chain in aheadend to convert one or more digital bit streams to an RF opticallymodulated signal.

FIG. 3 is a block diagram illustrating an existing overlay system.

FIG. 4 is a block diagram illustrating an improved based overlay system.

FIG. 5 is a block diagram illustrating an example implementation of thedigitizers in overlay system of FIG. 4.

FIG. 6 is a block diagram illustrating an example implementation of thefiber nodes in the overlay system of FIG. 4.

FIG. 7 is a block diagram illustrating an example implementation of theconverters in the fiber nodes in FIG. 6.

DETAILED DESCRIPTION

Various implementations of this disclosure modify existing overlaysystem architectures in a cost effective manner to meet the growingdemand for narrowcast services and to position the existing overlaysystems for additional future modifications. The implementations of theimproved overlay system of this disclosure re-digitize narrowcast analogsignals after they have been QAM modulated and upconverted to RFfrequencies and replace the analog narrowcast transmitters with digitalnarrowcast transmitters. In the fiber nodes, the received narrowcastsignals are converted back to analog RF modulated signals and combinedwith analog RF modulated broadcast signals for transmission to theservice groups.

Although this disclosure makes reference to a cable-based system, thisdisclosure is not intended to be limited to a cable-based system.

FIG. 1 illustrates an example cable-based system 100 operable to deliverhigh-definition digital entertainment and telecommunications such asvideo, voice, and high-speed Internet services over a cable network 130between a headend 110 and one or more service groups of receivingdevices such as cable modems (CMs) and/or settop boxes (STBs) 120(1), .. . , 120(n).

Analog video signals and digital bit streams representing variousservices (e.g., video, voice, and Internet) from various digitalinformation sources are received at the headend 110 and converted toradio frequency (RF) optically modulated signals for transmission overthe cable network 130. The cable network 130 can take the form of anall-coax, all-fiber, or hybrid fiber/coax (HFC) network. Traffictransferred from the headend 110 to a receiving device can be said totravel in a downstream direction; conversely, traffic transferred from areceiving device to the headend 110 can be said to travel in an upstreamdirection.

FIG. 2 illustrates an example processing chain 200 in the headend 110 toconvert one or more digital bit streams 205(1), . . . , 205(m) to an RFoptically modulated signal 265. For each digital bit stream 205(j), j=1,. . . , m, a channel coder 215(j) encodes the digital bit stream 205(j)to produce a corresponding digital quadrature amplitude modulation (QAM)symbol stream 220(j), for example, as specified in ITU-T RecommendationJ.83 (12/07), Annex B [ITU-T J.83-B], “Digital multi-programme systemsfor television sound and data services for cable distribution.”

Each of the digital QAM symbol streams 220(j), j=1, . . . , m, areconverted to an analog QAM symbol stream 235(j) by a digital to analog(D/A) converter 230(j). Each of the analog QAM symbol streams 235(j),j=1, . . . , m, is modulated onto an RF carrier signal having afrequency f_(j) that corresponds to a center frequency of a 6 MHz-wideRF channel by a QAM modulator and upconverter 240(j).

The analog single-channel modulated RF carrier signals 245(1), . . . ,245(m) can be combined by an RF combiner 250 to produce an analogmulti-channel RF signal 255. An analog optical transmitter 260 convertsthe analog multi-channel RF signal 255 to an RF optically modulatedsignals 265.

One or more of the processing blocks of the example processing chain 200can be re-arranged and/or eliminated and additional blocks can be addedto achieve a desired result. For example, the signals can be convertedto analog signals later in the chain, for example, after the channelsare combined.

FIG. 3 illustrates an existing overlay system 300. In the headend 310, abroadcast analog optical transmitter 305 receives an analog single ormulti-channel RF signal 303 (such as one of the analog single-channelmodulated RF carrier signals 245(j) or the analog multi-channel RFsignal 255 of FIG. 2). The analog broadcast transmitter 305 converts theanalog RF signal 303 to a broadcast RF optically modulated signal 307and transmits the broadcast RF optically modulated signal 307 downstreamon a first fiber 315.

For narrowcast services, for each fiber node 330(i), i=1 . . . , n inthe overlay system 300, there exists a corresponding narrowcast analogoptical transmitter 335(i) in the headend 310 to produce narrowcast RFoptically modulated signals designated for the fiber node 330(i). Eachof the narrowcast analog optical transmitters 335(i), i=1 . . . , nreceives an analog single or multi-channel RF signal 332(i) (such as oneof the analog single-channel modulated RF carrier signals 245(j) or theanalog multi-channel RF signal 255 of FIG. 2) and converts the RF signal332(i) to a narrowcast RF optically modulated signal 340(i) at aparticular wavelength λ_(i).

An optical multiplexer 345 multiplexes (e.g., via dense wavelengthdivision multiplexing) the narrowcast RF optically modulated signals340(1), . . . , 340(n) produced by the narrowcast analog opticaltransmitters 335(1), . . . , 335(n), respectively, to produce amulti-wavelength RF optically modulated signal 350 for transmission on asecond fiber 355.

The broadcast RF optically modulated signal 307 transmitted on the firstfiber 315 and the multi-wavelength RF optically modulated signal 350transmitted on the second fiber 355 can be received at an opticaltransition node (“OTN”) 360. At the OTN 360, the narrowcast signals aredemultiplexed by an optical demultiplexer 365. For each narrowcastsignal 340′(i), i=1, . . . , n (representing narrowcast RF opticallymodulated signal 340(i)) output from the demultiplexer 365, an opticalcombiner 370(i) optically combines the narrowcast signal 340′(i) and thebroadcast signal 307′ (representing broadcast RF optically modulatedsignal 307). The resulting signal 375(i) is transmitted to thedesignated fiber node 330(i).

Each of the fiber nodes 330(i), i=1, . . . , n includes an opticalreceiver that converts the received RF optically modulated signal375′(i) (representing signal 375(i)) to an electrical signal includingbroadcast and narrowcast services. The electrical signal then can betransmitted to receiving devices that are served by the fiber node330(i) (e.g., service group 320(i)). All of the receiving devices servedby the fiber node can receive the electrical signals. The portion of theelectrical signal representing a broadcast service is processed by eachreceiving device served by the fiber node and then the broadcast serviceis delivered to the subscriber. For the portion of the electrical signalrepresenting a narrowcast service, the receiving device to which theelectrical signal is addressed can process and deliver the correspondingnarrowcast service to the subscriber.

As discussed above, there is a growing demand for narrowcast services.Delivering more narrowcast content to meet the growing demand canrequire an increase in the number of narrowcast channels used by eachnarrowcast optical transmitter 335(i), i=1 . . . , n. Node segmentationalso can be used to deliver more narrowcast content to a fiber node.With node segmentation, additional narrowcast content is delivered to afiber node by transmitting narrowcast content at a plurality ofwavelengths designated for the fiber node. Thus, node segmentation canrequire more narrowcast optical transmitters 335(i), i=1 . . . , n toproduce narrowcast signals at the additional wavelengths for the fibernodes.

Due to various effects, such as fiber nonlinear intermodulation effects,the number of narrowcast channels that can be used by each narrowcastoptical transmitter 335(i), i=1 . . . , n can be limited. Furthermore,an increase in the number of wavelengths in the multi-wavelength RFoptically modulated signal 350 transmitted on the second fiber 355 canrequire additional EDFA amplifiers (not shown) along the fiber link 355between the headend 310 and the OTN 360 to preserve system performance.Due to these limitations, for example, existing overlay systemarchitectures, such as the overlay system 300 in FIG. 3, are notadequate to meet the growing demand for narrowcast services. It furthershould be noted that to maintain an appropriate relative signal level ofthe broadcast signal and narrowcast signal output from each of the fibernodes 330(i), the optical levels of the broadcast signal 307′ andnarrowcast signal 340′ (i) input to each combiner 370(i) must becarefully controlled. This can be costly.

Further, a substantial portion of the existing downstream fiber linksare in an overlay architecture similar to the overlay system 300 of FIG.3 with limited amounts of fiber capacity available to an OTN 360. Toincrease the number of fibers run to an OTN involves cost-prohibitiveoutside plant work to, for example, trench new fiber capacity. Thus, itcan be desirable to modify the existing architecture in a cost effectivemanner to meet the growing demand for narrowcast services and positionthe existing overlay systems for additional future modifications, forexample, to improve performance and meet future requirements such as therequirements of the converged media access platform (CMAP).

FIG. 4 illustrates an example implementation of an improved overlaysystem 400 to meet the growing demand for narrowcast services.

In the headend 410, a broadcast analog optical transmitter 405 receivesan analog single or multi-channel RF signal 403 (such as one of theanalog single-channel modulated RF carrier signals 245(j) or the analogmulti-channel RF signal 255 of FIG. 2). The analog broadcast transmitter405 converts the analog RF signal 403 to a broadcast RF opticallymodulated signal 407 and transmits the broadcast RF optically modulatedsignal 407 downstream on a first fiber 415.

Digitizers 433(i), i=1 . . . , n re-digitize the analog-modulated RFnarrowcast signals 432(i), i=1 . . . , n (e.g., one of the analogsingle-channel modulated RF carrier signals 245(j) or the analogmulti-channel RF signal 255 of FIG. 2), respectively, destined fornarrowcast transmitters. As discussed above with respect to FIG. 2, theanalog RF signals 432(i), i=1 . . . , n are derived from digital bitstreams that are QAM encoded and modulated. Because these RF signals432(i), i=1 . . . , n are derived from digital bit streams that areconverted to analog signals, among other reasons, it would not beobvious to one of ordinary skill in the art to re-digitize these RFsignals. FIG. 5 illustrates an example implementation of the digitizer433(i) in FIG. 4. A bandpass filter 505(i) filters the analog RF signal432(i) to filter out signals outside the frequency range for analog RFsignal 432(i). The resulting signal is converted to a digital signal byA/D converter 510(i). The digital signal can be filtered further bybandpass filter 515(i), downconverted to a baseband signal by a digitalmixer 520(i), and low pass filtered by filter 525(i) to produce adigitized signal 434(i). This disclosure is not limited to anyparticular digitizer. Any existing or future developed digitizer isintended to be included within the scope of this disclosure.

Referring back to FIG. 4, the digitized RF signals 434(i), i=1 . . . , nare received by narrowcast digital optical transmitters 435(i), i=1 . .. , n, which convert the digitized RF signals to narrowcast RF opticallymodulated signals 440(i), i=1 . . . , n at particular wavelength λ_(i),i=1, . . . n, respectively.

An optical multiplexer 445 combines (e.g., via dense wavelength divisionmultiplexing) the narrowcast RF optically modulated digital signals440(i), . . . , 440(n) produced by the narrowcast digital opticaltransmitters 435(1), . . . , 435(n), respectively, to produce amulti-wavelength RF optically modulated signal 450 for transmission on asecond fiber 455.

The broadcast RF optically modulated signal 407 transmitted on the firstfiber 415 and the multi-wavelength RF optically modulated signal 450transmitted on the second fiber 455 can be received at OTN 460. At theOTN 460, the narrowcast signals can be demultiplexed by an opticaldemultiplexer 465. For each narrowcast signal 440′(i), i=1, . . . , n(representing narrowcast RF optically modulated signal 440(i)) outputfrom the demultiplexer 465, an optical multiplexer 470(i) multiplexes(e.g., via dense wavelength division multiplexing) the narrowcast signal440′(i) and the broadcast signal 407′ (representing broadcast RFoptically modulated signal 407). The resulting signal 475(i) transmittedto the designated fiber node 430(i).

Each of the fiber nodes 430(i), i=1, . . . , n converts the received RFoptically modulated signals 475′(i), i=1, . . . , n (representing signal475(i)) to electrical signals including broadcast and narrowcastservices. The electrical signals 480(i) then are transmitted to thecorresponding service group 420(i).

FIG. 6 illustrates an example implementation of the fiber nodes 430(i).In each of the fiber nodes 430(i), i=1, . . . , n, the multiplexedsignal 475′(i) is demultiplexed by an optical demultiplexer 605(i) toproduce a broadcast signal 407″ that represents the broadcast signal407′(i) and a narrowcast signal 440″(i) that represents the narrowcastsignal 440′(i). A receiver 607(i) extracts the broadcast RF-modulatedelectrical signal 609(i) from the optical broadcast signal 407″(i)received from the demultiplexer 605(i). The narrowcast digital opticalsignal 440″(i) is converted to an analog RF-modulated electrical signal615(i) by converter 610(i) and then the analog narrowcast signal 615(i)and the analog broadcast signal 609(i) are electrically combined bycombiner 620(i) to produce electrical signal 480(i).

FIG. 7 illustrates an example implementation of the converter 610(i) inFIG. 6. A receiver 705(i) extracts the digital signal 707(i) from theoptical narrowcast signal 440″(i) received from the demultiplexer 605(i)of FIG. 6. An upconverter 710(i) frequency shifts the digital signal707(i) to the appropriate channel frequency and the resulting signal isconverted to an analog signal 615(i) by D/A converter 715.

By re-digitizing the narrowcast analog RF signals (such as one of theanalog single-channel modulated RF carrier signals 245(j) or the analogmulti-channel RF signal 255 of FIG. 2) and replacing the analognarrowcast transmitters 335(i), i=1 . . . , n with digital narrowcasttransmitters 435(i), i=1 . . . , n, the number of narrowcast channelsper narrowcast transmitter may increase due to the reduced signal tonoise requirements of digital transmission technologies. Furthermore, anincrease in the number of wavelengths in the multi-wavelength RFoptically modulated signal 450 transmitted on the second fiber 455 maynot require additional EDFA amplifiers along the fiber link 455 betweenthe headend 410 and the OTN 460 to preserve system performance. Thus,the improved overlay system 400 may meet the growing demand fornarrowcast services without requiring expensive plant rebuilds.

Furthermore, unlike in the existing overlay system architectures, in theoverlay systems according to the present disclosure, the signal level ofthe narrowcast signals output from each of the fiber nodes 430(i) areindependent of the optical level of the narrowcast signals 440′(i),440″(i). This independence results from the narrowcast signals outputfrom each of the fiber nodes 430(i) being reconstructed from digitaldata streams. Thus, provided the optical levels are sufficient toprevent data errors, the variation in the optical level of thenarrowcast signals 440′(i), 440″(i) should not affect the signal levelof the narrowcast signals output from each of the fiber nodes 430(i).Although the signal level of the broadcast signals output from each ofthe fiber nodes 430(i) can depend on the optical level of the broadcastsignals 407′(i), 407″(i), this independence can be acceptable since atleast the signal level of the narrowcast signals output from each of thefiber nodes 430(i) are independent of the optical level of thenarrowcast signals 440′(i), 440″(i). Thus, unlike the existing overlaysystem architectures, the optical levels of the broadcast signals 407′,407″ and narrowcast signals 440′(i), 404″(i) may not require carefullycontrol to maintain an acceptable relative signal level of the broadcastsignals and narrowcast signals output from each of the fiber nodes430(i). Thus, the cost overlay systems according to the presentdisclosure can be lower than existing overlay system architectures.

Nevertheless, in some implementations, the optical level of thebroadcast signal (e.g., broadcast signal 407″(i)) can be monitored basedon known and future developed methods to maintain an appropriate signallevel for output signal level 609(i). For example, the receiver 607(i)can be equipped with an optical input level monitor to monitor theoptical level of the broadcast signal 407″(i). When the optical levelchanges, the receiver 607(i) can adjust its gain to maintain a constantoutput signal level for broadcast signal 609(i). As another example, thereceiver 607(i) can implement an automatic gain control loop to maintaina constant output signal level for broadcast signal 609(i).

However, if there is a need to change the relative signal level of thebroadcast signals and narrowcast signals output from each of the fibernodes 430(i) (for example, due to a change in system broadcast andnarrowcast loading), the existing overlay system architectures requiretruck-rolls to system nodes to adjust system components. The relativesignal level of the broadcast signals and narrowcast signals output fromeach of the fiber nodes 430(i) can be more efficiently adjusted inoverlay systems according to the present disclosure. For example, insome implementations, data instructing the converter 610(i) to adjustthe output signal level of the narrowcast signal 615(i) and/orinstructing the receiver 607 to adjust the output signal level of thebroadcast signal 609(i) can be embedded in the narrowcast signals 440(i)and extracted by the converter 610(i). The converter 610(i) can providea signal 611(i) to receiver 607 to adjust the output signal level ofbroadcast signal 609(i). In this way, the relative signal level of thebroadcast signals and narrowcast signals output from each of the fibernodes 430(i) can be adjusted remotely from a headend and may eliminatethe need for truck-rolls to the field thereby resulting in cost savings.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or of what may be claimed, but rather as descriptions offeatures that may be specific to particular implementations ofparticular inventions. Certain features that are described in thisspecification in the context of separate implementations can also beimplemented in combination in a single implementation. Conversely,various features that are described in the context of a singleimplementation can also be implemented in multiple implementationsseparately or in any suitable subcombination. Moreover, althoughfeatures may be described above as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a subcombination or variation ofa subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Particular implementations of the subject matter described in thisspecification have been described. Other implementations are within thescope of the following claims. For example, the actions recited in theclaims can be performed in a different order and still achieve desirableresults, unless expressly noted otherwise. As one example, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In some implementations, multitasking and parallel processingmay be advantageous.

The invention claimed is:
 1. A transmitting headend or transmitting nodein an overlay transmission system for transmitting broadcast andnarrowcast services, the transmitting headed or transmitting node in theoverlay transmission system comprising: an analog optical transmitterand a plurality of narrowcast digital optical transmitters overlaid fortransmissions to a receiving node, the analog optical transmitterextending in a downstream direction toward the receiving node via afirst fiber and the plurality of narrowcast digital optical transmittersextending in a downstream direction toward the receiving node via asecond fiber; the analog optical transmitter configured for converting areceived analog signal to a broadcast radio frequency (RF) opticallymodulated signal for transmission on the first fiber; a plurality ofdigitizers each configured to: receive an analog signal derived from oneor more digital bit streams that each have been QAM encoded andmodulated; and for transmission to the plurality of narrowcast digitaloptical transmitters, re-digitize the analog signal derived from the oneor more digital bit streams, wherein re-digitizing by the plurality ofdigitizers produces a plurality of digital bit streams for transmissionby the plurality of narrowcast digital optical transmitters to thereceiving node for maintaining independence of an optical level of there-digitized signal received from the respective digitizer; theplurality of narrowcast digital optical transmitters configured toreceive the plurality of digital bit streams from the plurality ofdigitizers, respectively, and configured to convert each of theplurality of digital bit streams to a digital narrowcast signaloptically modulated at a respective wavelength for digital transmissionon the second fiber; and a optical multiplexer configured to multiplexthe plurality of narrowcast optically modulated signals to produce amulti-wavelength digitally modulated optical signal for the digitaltransmission on the second fiber.
 2. The overlay system of claim 1wherein the multiplexer comprises a dense wavelength divisionmultiplexer.
 3. A method of digitizing a forward path in an overlaysystem, the method comprising: receiving at least one analog signal forconversion to a broadcast signal for transmission to a node; convertinga received analog signal to a broadcast radio frequency (RF) opticallymodulated signal for transmission on a first fiber; receiving aplurality of analog signals wherein each analog signal is derived fromone or more digital bit streams that have been QAM encoded andmodulated; re-digitizing each of the plurality of analog signals derivedfrom the one or more digital bit streams that have been QAM encoded andmodulated to produce corresponding re-digitized signals, whereinre-digitizing each of the plurality of analog signals produces aplurality of digital bit streams for transmission by a plurality ofnarrowcast digital optical transmitters to a receiving node formaintaining independence of an optical level of the re-digitizedsignals; converting each of the plurality of digital bit streams to adigital narrowcast signal optically modulated at a respective wavelengthfor digital transmission on the second fiber; and multiplexing theplurality of narrowcast optically modulated signals to produce amulti-wavelength digitally modulated optical signal for the digitaltransmission on the second fiber to the node.
 4. The method of claim 3wherein multiplexing the plurality of optical signals comprises densewavelength division multiplexing the plurality of optical signals.